It has been reported that the imide compound hydrochloride of the above formula (2) can be produced by treating an imide compound in free form of the formula (1):
with a hydrogen chloride 2-propanol solution in acetone, and crystallizing the resultant. However, said process is not sufficient enough for an industrial process from the aspect of the availability and the handling of the reagents to be used therein (cf., JP-A-5-17440).